World population: 6,775,235,700
Population of NYC: 8,364,000
If the entire world population migrated into large cities we could fit all of the world into 810 cities of NYC's population.
If all of those cities were the population density of Manila, at 111,576 people per square mile, then we could fit onto .09% of the entire Earth's land surface.
Imagine having only 810 airports to maintain, only 810 cities connected by high speed railroads, only 810 massive public transit projects, virtually no need for cars, and 99.9% of land untouched or farmland.